Alloyed molten zinc plated steel sheet and process of production of same

ABSTRACT

The present invention provides an alloyed molten zinc plated steel sheet having an area of the Fe and Zn alloy phase in the unformed parts in the plating layer of less than 10% of the area of the steel sheet as a whole and superior in strength and shapeability and a method of producing this alloyed molten zinc plating steel sheet by a continuous zinc plating production system which enables production at a low cost without modification of the system or addition of steps, said alloyed molten zinc plated steel sheet characterized by comprising a steel sheet including C: 0.05 to 0.40%, Si: 0.2 to 3.0%, and Mn: 0.1 to 2.5%, the balance comprised of Fe and unavoidable impurities, having on its surface a Zn alloy plating layer comprised of Fe in a concentration of 7 to 15 wt %, Al in a concentration of 0.01 to 1 wt %, and the balance of Zn and unavoidable impurities, said plating layer containing oxide particles of at least one type of oxide selected from an Al oxide, Si oxide, Mn oxide, and complex oxides of the same alone or in combination.

TECHNICAL FIELD

The present invention relates to a high strength, alloyed molten zincplated steel sheet able to be utilized as a member of an automobile,building material, or electrical appliance and a process of productionof the same.

BACKGROUND ART

In the auto industry, demand has been rising for steel sheet providedwith the properties of both shapeability and high strength so as toachieve both lighter weight of the chassis to deal with environmentalproblems and safety in collisions.

To deal with these needs, Japanese Unexamined Patent Publication (Kokai)No. 5-59429 discloses steel sheet having as the steel sheet structure amixture of the three phases of the ferrite phase, bainite phase, andaustenite phase and transforming the residual austenite to martensite atthe time of shaping so as to utilize the transformation-inducedplasticity exhibiting a high ductility. This type of steel sheet forexample forms a complex structure by the addition, by wt %, of C: 0.05to 0.4%, Si: 0.2 to 3.0% A, and Mn: 0.1 to 2.5% in the steel andcontrolling the temperature pattern in the process of annealing in thetwo-phase region, then cooling and is characterized in that the desiredproperties can be brought out without the use of expensive alloyelements.

When zinc plating this steel sheet by a continuous molten zinc platingsystem, usually the surface of the steel sheet is degreased, the surfaceis cleaned, then, for the purpose of forming the above-mentionedstructure, the sheet is heated in an nonoxidizing furnace to form aniron oxide layer of a thickness of 50 nm to 1 μm or so on the surface ofthe steel sheet, annealing the sheet in a reducing furnace to reduce theiron oxide layer, then dipping the sheet in a molten zinc plating bathto plate it with zinc. When producing an alloyed molten zinc platedsteel sheet, the steel sheet is dipped in a plating bath in that step,then held at a temperature of 400 to 600° C. or so to alloy the zinc andiron and convert the plating layer to an alloy phase of Fe and Znconstituting an δ1 phase.

Steel sheet, however, contains large amounts of easily oxidizingelements such as Si and Mn compared with the ordinary deep drawncold-rolled steel sheet etc., so there is the problem that the surfaceof the steel sheet is easily formed with Si oxides, Mn oxides, or Si andMn complex oxides in the heat treatment performed in the above series ofsteps. However, in industrial scale systems, it is difficult to reducethe oxygen potential of the atmosphere in the heating step to an extentwhere Si or Mn will not be oxidized, so formation of Si and Mn oxides atthe surface of the steel sheet is substantially unavoidable. Further, ifthe surface of the steel sheet is formed with an Si oxide layer or Mnoxide layer, there is the problem that the alloying of the Zn and Fe isinhibited in the alloying step at the time of production of the alloyedmolten zinc plated steel sheet and parts where the Fe—Zn alloy phasehave not yet been formed remain.

One method easily conceivable as a means for solving these problems isto set the alloying treatment temperature slightly high to promotealloying of Fe and Zn. At the alloying treatment temperature of 450 to600° C., however, austenitic transformation occurs in the steel sheet,so if setting the alloying treatment temperature slightly high,depending on the holding time, the structure of the steel sheet will notbecome the desired mixed structure of a mixture of the three phases ofthe ferrite phase, bainite phase, and austenite phase. As a result,there is the problem that the shapeability and strength of the steelsheet aimed at cannot be secured in some cases.

To deal with this problem, Japanese Unexamined Patent Publication(Kokai) No. 55-122865 discloses the method of forming a 40 to 1000 nmiron oxide layer on the surface of a steel sheet in a heat treatmentstep by a nonoxidizing furnace in a continuous molten zinc plating stepso as to prevent outward diffusion of the Si or Mn in the reductionstep, suppress the formation of the Si oxide layer, and improve theplating properties. With this method, however, if the reduction time istoo long for the thickness of the iron oxide layer, Si will become denseat the surface of the steel sheet and an Si oxide layer will be formed,while if the reduction time is too short, iron oxide will remain on thesurface of the steel sheet and defects in the plating properties, thatis, the formation of unformed parts of the Fe—Zn alloy phase will beformed. Further, in recent continuous molten zinc plating systems,annealing systems using radiant type heating furnaces rather thannonoxidizing furnaces are becoming the mainstream. In such systems,there was the problem that the above method could not be used.

Further, Japanese Unexamined Patent Publication (Kokai) No. 2000-309824discloses as a method for preventing selective oxidation of the Si or Mnat the time of annealing the method of hot rolling the steel sheet, thenheat treating it in the state with the black skin scale still attachedin an atmosphere where reduction will substantially not occur and in atemperature range of 650 to 950° C. so as to form a sufficient internaloxide layer in the base iron surface layer. With this method, however,in addition to the conventional continuous molten zinc plating step, aheat treatment step for forming the internal oxide layer and a picklingtreatment step become necessary, so there was the problem that a rise inproduction costs was invited. Further, the plated steel sheet having theinternal oxide layer had the problem of easily peeling of the platinglayer.

DISCLOSURE OF INVENTION

In view of the above problems, the present invention has as its objectthe provision of an alloyed molten zinc plated steel sheet wherein thearea of the unformed parts of the Fe—Zn alloy phase in the plating layeris less than 10% of the area of the steel sheet as a whole and whereinthe strength and shapeability are superior. Further, it has as itsobject the provision of a process of production of the alloyed moltenzinc plated steel sheet at a low cost without modifying the system oradding steps in a conventional continuous molten zinc plating productionsystem.

To solve the above problem, the inventors engaged in intensive studiesand as a result newly discovered that by including in the plating layeroxide particles of at least one type selected from an Al oxide, Sioxide, Mn oxide, Al and Si complex oxide, Al and Mn complex oxide, Siand Mn complex oxide, and Al, Si, and Mn complex oxide alone or incombination, alloying of the plating layer is promoted and uniformalloying across the entire surface of the steel sheet is obtained andmade it possible to provide an alloyed molten zinc plating steel sheetwherein the area of the unformed parts of the Fe—Zn alloy phase in theplating layer is less than 10% of the area of the steel sheet as a wholeand wherein the strength and shapeability are superior.

The fundamental reason why addition of oxide particles in the platinglayer causes alloying of the plating layer to be promoted and a uniformalloy layer to be obtained across the entire steel sheet is unclear, butthe inventors continued with their intensive studies and as a resultdiscovered that by making the plating layer the above structure, thealloying of Fe—Zn occurs uniformly across the entire surface of thesteel sheet.

Further, the inventors discovered that the above alloyed molten zincplated steel sheet can be obtained by adjusting the ratio PH₂O/PH₂ ofthe steam partial pressure and hydrogen partial pressure of theatmosphere in the reducing furnace in the recrystallization annealingstep of a continuous molten zinc plating system to1.4×10⁻¹⁰T²−1.0×10⁻⁷T+5.0×10⁻⁴ to 6.4×10⁻⁷T²+1.7×10⁻⁴T−0.1 with respectto the heating temperature T (° C.), forming internal oxide at a regionfrom the surface of the steel sheet to a depth of 1.0 μm, thensuccessively performing molten zinc plating treatment and alloyingtreatment. The present invention has the following as its gist:

(1) An alloyed molten zinc plated steel sheet characterized bycomprising a steel sheet including, by wt %,

-   -   C: 0.05 to 0.40%,    -   Si: 0.2 to 3.0%, and    -   Mn: 0.1 to 2.5% and    -   further including at least one or two or more types of:    -   P: 0.001 to 0.05%,    -   S: 0.001 to 0.05%,    -   Al: 0.01% to 2%,    -   B: 0.0005% to less than 0.01%,    -   Ti: 0.01% to less than 0.1%,    -   V: 0.01% to less than 0.3%,    -   Cr: 0.01% to less than 1%,    -   Nb: 0.01% to less than 0.1%,    -   Ni: 0.01% to less than 2.0%,    -   Cu: 0.01% to less than 2.0%,    -   Co: 0.01% to less than 2.0%,    -   Mo: 0.01% to less than 2.0%,    -   with the balance comprised of Fe and unavoidable impurities,        having on its surface a Zn alloy plating layer comprised of Fe        in a concentration of 7 to 15 wt %, Al in a concentration of        0.01 to 1 wt %, and the balance of Zn and unavoidable        impurities, said plating layer containing oxide particles of at        least one type of oxide selected from an Al oxide, Si oxide, Mn        oxide, Al and Si complex oxide, Al and Mn complex oxide, Si and        Mn complex oxide, and Al, Si, and Mn complex oxide alone or in        combination.

(2) An alloyed molten zinc plated steel sheet as set forth in (1),characterized in that said oxide particles are comprised of at least oneof silicon oxide, manganese oxide, aluminum oxide, aluminum silicate,manganese silicate, manganese aluminum oxide, and manganese aluminumsilicate.

(3) An alloyed molten zinc plated steel sheet as set forth in (1),characterized in that an average diameter of the particle size of saidoxide is 0.01 to 1 μm.

(4) An alloyed molten zinc plated steel sheet as set forth in any one of(1) to (3), characterized in that the structure of said steel sheet hasa complex structure of a ferrite phase, bainite phase, and residualaustenite phase.

(5) A process of production of an alloyed molten zinc plated steel sheetcomprised of the ingredients described in (1) by a continuous moltenzinc plating system, said process of production of an alloyed moltenzinc plated steel sheet characterized by making a heating temperature Tat a recrystallization annealing step in a reducing furnace of saidsystem 650° C. to 900° C., passing the steel sheet through an atmospherewhere a ratio PH₂O/PH₂ of the steam partial pressure PH₂O and hydrogenpartial pressure PH₂ of the atmosphere of said reducing furnace is1.4×10⁻¹⁰T²−1.0×10⁻⁷T+5.0×10⁻⁴ to 6.4×10⁻⁷T²+1.7×10⁻⁴T−0.1, forminginternal oxide at a region from the surface of the steel sheet to adepth of 1.0 μm, then successively performing molten zinc platingtreatment and alloying treatment.

(6) A process of production of an alloyed molten zinc plated steel sheetas set forth in (5), characterized in that said oxide particles arecomprised of at least one of silicon oxide, manganese oxide, aluminumoxide, aluminum silicate, manganese silicate, manganese aluminum oxide,and manganese aluminum silicate.

(7) A process of production of an alloyed molten zinc plated steel sheetas set forth in (5), characterized in that an average diameter of theparticle size of said oxide is 0.01 to 1 μm.

(8) A process of production of an alloyed molten zinc plated steel sheetas set forth in any one of (5) to (7), characterized in that thestructure of said steel sheet has a complex structure of a ferritephase, bainite phase, and residual austenite phase.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic view of an example of the cross-section of analloyed molten zinc plated steel sheet of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

The alloyed molten zinc plated steel sheet of the present invention ischaracterized by being provided with both a superior press formabilityand strength and by having an area occupied by the parts where the Fe—Znalloy phase is not formed in the plating layer of less than 10% of thearea of the steel sheet as a whole.

To impart this characterizing feature, first, to secure the ductilityand strength of the steel sheet itself, the ingredients of the steelsheet are made, by wt %, C: 0.05 to 0.40%, Si: 0.2 to 3.0%, Mn: 0.1 to2.5%, and the balance of Fe and unavoidable impurities, while thestructure of the steel sheet is made a complex phase structure includingthe ferrite phase, bainite phase, and austenite phase. Note that thecontents of the steel composition defined in the present invention areall wt %.

The reasons for addition of the additive elements to the steel sheetbase material of the alloyed molten zinc plated steel sheet used in thepresent invention will be explained below.

C is an element added for stabilizing the austenite phase of the steelsheet. If the content of the C is less than 0.05%, its effect cannot beexpected. Further, if over 0.40%, the bondability is degraded and adetrimental effect is given when actually using the molten zinc platedsteel sheet of the present invention, so the content is made 0.05% to0.4%.

Si is an element required when creating a stable presence of anaustenite phase even at room temperature due to the action of increasingthe concentration of C in the austenite phase. If the content is lessthan 0.2%, its effect cannot be expected, while if over 3.0%, theinternal oxide film is formed thickly—inviting peeling of the plating,so the content of Si is made 0.2% to 3.0%.

Mn is an element required for preventing the austenite from transformingto pearlite in the heat treatment step. If the content is less than0.1%, its effect is nonexistent, while if over 2.5%, the bonded partsbreak and there are other detrimental effects in actual use of themolten zinc plated steel sheet of the present invention, so theconcentration of the Mn is made 0.1% to 2.5%.

The steel sheet base material of the present invention basicallycontains the above elements, but the added elements are not limited tojust these elements. It is also possible to include elements alreadyknown to have the effect of improvement of the properties of the steelsheet, for example, Al having the effect of improving the pressformability. The amount of Al required for improving the pressformability of steel sheet is preferably at least 0.01%. Excessiveaddition of Al would invite degradation of the plating properties and anincrease in inclusions, so the content of Al is preferably not more than2%.

Further, it is possible to add P:0.001 to 0.05% and S:0.001 to 0.05%. Pis an element required for strengthening the steel in an amount inaccordance with the required strength. If the excess amount of P isadded, P segregates at grain boundaries and deteriorates elongation.Therefore, the upper limit of the P addition is preferable limited to0.05%. On the other hand, the lower limit of the P addition ispreferable limited to 0.001% because of considering the increase of therefining cost in the steel making process.

S is an unfavorable element for deteriorating local elongation andweldability of the steel because of forming MnS. Therefore, the upperlimit of the S addition is preferable limited to 0.05%. On the otherhand, the lower limit of the S addition is preferable limited to 0.001%because of considering the increase of the refining cost in the steelmaking process as the same reason as P.

Further, for example, it is also possible to add one or two or more ofB, Ti, V, Cr, and Nb having the effect of improvement of quenching in anamount of B of 0.0005% to less than 0.01%, Ti of 0.01% to less than0.1%, V of 0.01% to less than 0.3%, Cr of 0.01% to less than 1%, and Nbof 0.01% to less than 0.1%. These elements are added with theexpectation of improving the quenchability of the steel sheet, so ifless than the above contents, no effect of improvement of thequenchability can be expected. Further, inclusion in an amount over theupper limit of the above content is possible, but the effect becomessaturated and an effect of improvement of quenchability commensuratewith the cost can no longer be expected.

Further, for example, it is also possible to include Ni, Cu, Co, Mo, andother elements having the effect of improvement of strength in amountsof 0.01% to less than 2.0%. These elements are added in the expectationof the effect of improvement of strength. On the other hand, anexcessive content of Ni, Cu, Co, or Mo leads to excessive strength or arise in the alloy costs. Further, the sheet may also contain N and othergenerally unavoidable elements.

The molten zinc plated steel sheet of the present invention is made acomplex phase structure comprised of the three phases of a ferritephase, austenite phase, and bainite phase in order to impart superiorprocessability and strength by processing-induced transformation at roomtemperature.

The composition of the plating layer of the alloyed molten zinc platedsteel sheet according to the present invention is made, by wt %, aconcentration of Fe of 7 to 15%, a concentration of Al of 0.01 to 1%,and a balance of Zn and unavoidable impurities.

The reason is that, for Fe, if the concentration of Fe of the platinglayer is less than 7%, chemical conversion treatment becomes poor, whileif over 15%, peeling of the plating occurs due to the processing. ForAl, if the content of Al in the plating layer is less than 0.01%, thealloying of Fe and Zn becomes excessive, while if over 1%, the corrosionresistance is degraded. Further, the basis weight of the plating is notparticularly limited.

Next, the structure of a plating layer of the alloyed molten zinc platedsteel sheet of the present invention will be explained.

FIG. 1 shows an example of a schematic view of the cross-section of analloyed molten zinc plated steel sheet of the present invention. Thealloyed molten zinc plated steel sheet of the present invention is of astructure containing at least one of particles of Al oxide, Si oxide, Mnoxide, Al and Si complex oxide, Al and Mn complex oxide, Si and Mncomplex oxide, and Al, Si, and Mn complex oxide contained in the platinglayer alone or in combination. By making the plating layer such astructure, alloying of the Fe and Zn is promoted by the oxide particlesin the plating layer, uniform alloying occurs across the entire surfaceof the steel sheet, and the parts where the Fe—Zn alloy phase is notformed become less than 10% of the area of the steel sheet as a whole.

The extent of alloying of Fe—Zn of the plating layer is evaluated byrandomly selecting analysis points from a steel sheet, assaying theingredients of the plating layer, and judging cases where thecomposition of the plating layer is in the range of the presentinvention, that is, where the concentration of Fe is in the range of 7to 15 wt %, as passing. The analysis method is not particularly limited.The following examples of the analysis method and evaluation do notlimit the present patent either. As the analysis method, for example, itis possible to use the method of assaying the concentration of Fe in theplating layer by glow discharge optical emission spectrometry,fluorescent X-ray analysis, X-ray microanalysis, or transmissionelectron microscope or of chemically analyzing the plating layer bydissolving it in a solution. The size of each analysis point should beset to the optimal size in accordance with the analysis method used.Further, the number of analysis points per steel sheet is also notlimited, but to obtain very representative evaluation results, aplurality of locations are analyzed for one steel sheet and it isconfirmed that the locations where the composition of the plating layeris in the range of the present invention, that is, where theconcentration of Fe is in the range of 7 to 15 wt %, account for atleast 90% of the total analyzed locations. For this purpose, as thenumber of analysis points, it is desirable to analyze at least fivelocations randomly selected for a steel sheet.

For example, it is possible to use the following method of evaluation.That is, the extent of alloying of Fe—Zn of the plating layer isevaluated by randomly selecting 10 analysis points from a steel sheetand assaying the concentration of Fe in the plating layer by glowdischarge optical emission spectrometry. At this time, the size of eachanalysis point is made a constant diameter of 5 mm. Cases where at leastnine locations having concentrations of Fe in the plating layer of 7 to15 wt % are judged as passing and other cases are judged as failing.Cases where there are two or more locations where the concentration ofFe in the plating layer is less than 7 wt % are judged as beinginsufficiently alloyed and as therefore failing, while cases where thereare two or more locations where the concentration is over 15 wt % arejudged as being excessively alloyed.

The Al oxide, Si oxide, Mn oxide, Al and Si complex oxide, Al and Mncomplex oxide, Si and Mn complex oxide, and Al, Si, and Mn complex oxidecontained in the plating layer are respectively silicon oxide, manganeseoxide, aluminum oxide, aluminum silicate, manganese silicate, manganesealuminum oxide, and manganese aluminum silicate. Si, Mn, and Al areelements added as ingredients of the steel sheet. These become oxides atthe surface layer of the steel sheet in the heat treatment step of thesteel sheet. They can be easily included in the plating layer forforming silicon oxide, manganese oxide, aluminum oxide, aluminumsilicate, manganese silicate, manganese aluminum oxide, and manganesealuminum silicate. The method for including the oxide particles in theplating layer will be explained later.

Note that the oxide particles to be contained in the plating layer topromote the alloying of Fe and Zn of the plating layer may also beoxides other than the above silicon oxide, manganese oxide, aluminumoxide, aluminum silicate, manganese silicate, manganese aluminum oxide,and manganese aluminum silicate, but in this case the oxide particleshave to be added to the plating bath or the main ingredient elements ofthe oxides have to be added to the steel sheet—inviting a rise of theproduction costs.

The size of the oxide particles contained in the plating layer ispreferably an average diameter of 0.01 μm to 1 μm. The reason is that ifthe average diameter of the oxide particles is less than 0.01 μm, theeffect of causing uniform alloying of Fe—Zn in the plating layer falls.If making the average diameter of the oxide particles more than 1 μm, atthe time of processing the alloyed molten zinc plated steel sheet, theoxide particles easily become starting points of fracture and thecorrosion resistance of the processed parts is degraded, that is,detrimental effects easily occur when putting the molten zinc platedsteel sheet into practical use.

Note that the “average diameter” of the oxide particles referred to inthe present invention indicates the average equivalent circular diameterof the oxide particles detected by observation of the cross section ofthe plating layer. The shape of the oxide particles may be spherical,plate-like, or conical.

As the method of measuring the average diameter of the oxide particles,the method may be mentioned of polishing the cross section of thealloyed molten zinc plated steel sheet or using FIB (focused ion beamprocessing system) to process the sheet to expose the cross section andthereby prepare a sample, then analyzing it by observation by a scanelectron microscope, plane analysis by X-ray microanalysis, or planeanalysis by Auger electron spectroscopy. Further, it is possible toprocess the cross section of the steel sheet to a thin piece so as toinclude the plating layer, then observe this by a transmission typeelectron microscope. In the present invention, the image data obtainedby these analysis methods is analyzed to calculate the equivalentcircular diameter of the oxide particles. The average value should be0.01 μm to 1 μm. Particles of less than 0.01 μm and particles of morethan 1 μm may also be included in the observed region.

Further, the content of the oxide particles in the plating layer is notparticularly limited, but preferably the plating layer contains theparticles in a density of 1×10⁸ particles/cm² to 1×10¹¹ particles/cm².If the content of the oxide particles is less than 1×10⁸ particles/cm²,sometimes the effect of the alloying of the Fe and Zn of the platinglayer being promoted and the uniform alloying occurring across theentire surface of the steel sheet cannot be expected. On the other hand,excess oxide particles of over 1×10¹¹ particles/cm² become a cause ofpeeling of the plating layer.

Next, the process of production of the alloyed molten zinc plated steelsheet of the present invention will be explained.

In the present invention, a continuous molten zinc plating system isused for alloyed molten zinc plating of the above high strength steelsheet.

In the process of production of an alloyed molten zinc plated steelsheet of the present invention, the heating pattern is set so that thesteel sheet becomes the above desired structure in the recrystallizationannealing step of the continuous molten zinc plating system. That is, areducing furnace is used to anneal steel sheet in a two-phase coexistingregion of 650 to 900° C. for 30 seconds to 10 minutes. The atmosphere inthe reducing furnace is made a nitrogen gas including hydrogen gas in arange of 1 to 70 wt %. The inside of the furnace is adjusted to a ratio(PH₂O/PH₂) of the steam partial pressure and hydrogen partial pressureof the atmosphere by introducing steam. In the present invention, theratio PH₂O/PH₂ of the steam partial pressure and hydrogen partialpressure of the atmosphere of the reducing furnace is adjusted to1.4×10⁻¹⁰T²−1.0×10⁻⁷T+5.0×10⁻⁴ to 6.4×10⁻⁷T²+1.7×10⁻⁴T−0.1 with respectto the heating temperature T (° C.) in the recrystallization annealingstep.

The reason for limiting the ratio PH₂O/PH₂ of the steam partial pressureand hydrogen partial pressure of the atmosphere of the reducing furnaceto the above range is as follows. That is, in the present invention,since the steel sheet contains Si in an amount of at least 0.2 wt % andMn in at least 0.1 wt %, if PH₂O/PH₂ is less than1.4×10⁻¹⁰T²−1.0×10⁻⁷T+5.0×10⁻⁴, an external oxide film is formed on thesurface of the steel sheet and poor bonding of the plating occurs.Further, in the present invention, the Si added to the steel sheet isnot more than 3.0 wt % and Mn not more than 2.5 wt %, so if PH₂O/PH₂exceeds 6.4×10⁻⁷T²+1.7×10⁻⁴T−0.1, fayalite and other Fe oxides areformed and plating gaps arise. By annealing by the above method, it ispossible to form a region from the surface of the steel sheet to a depthof 1.0 μm with a structure having least one type of internal oxide ofsilicon oxide, manganese oxide, aluminum oxide, aluminum silicate,manganese silicate, manganese aluminum oxide, and manganese aluminumsilicate alone or in combination.

Next, in the plating step, the steel sheet is cooled at a cooling rateof 2 to 200° C. per second to a temperature range of 250 to 500° C.,held there for 5 seconds to 20 minutes, then plated by being dipped in amolten zinc plating bath comprised of Al in an amount of 0.01 wt % to 1wt % with the balance of Zn and unavoidable impurities. The temperatureand dipping time of the plating bath at this time are not particularlylimited. Further, the example of the heating and cooling patterns in theplating step does not limit the present invention.

After the above molten zinc plating, in the alloying step, the steelsheet is held at a temperature of 450 to 600° C. for 5 seconds to 2minutes to cause an alloying reaction of Fe and Zn and to cause theinternal oxide formed at the surface of the steel sheet at the annealingstep in the reducing furnace to migrate to the plating layer to form thecharacteristic of the alloyed molten zinc plated steel sheet of thepresent invention, that is, the plating layer structure containing oxideparticles in a plating layer.

In the case of forming the above mentioned plating layer structure, alloxide particles formed at the surface of the steel sheet do not alwaysmove into the plating layer, but some of the oxide particles may remainin the steel sheet.

In the present invention, Fe and Zn alloying is promoted by the actionof the oxide particles contained in the plating layer. If the heatingtemperature and holding time are in the above range in the alloyingstep, sufficiently uniform alloying is possible. Therefore, it ispossible to finish the alloying treatment while the austenite phase inthe steel sheets is not reduced. Consequently, steel sheets having thedesired mixed structures of the ferrite phase, bainite phase, andaustenite phase can be obtained.

EXAMPLES

Below, the present invention will be explained in detail by examples,but the present invention is not limited to these examples.

The test steel sheets shown in Table 1 were treated forrecrystallization annealing, plating, and alloying by a continuousmolten zinc plating system in accordance with the conditions shown inTable 2.

TABLE 1 Test material Composition (wt %) code C Si Mn Al P S Ti Nb Ni CuRemarks NA 0.1 1.2 1.3 0.004 0.003 Invention A 0.1 0.2 1.6 0.1 0.0050.006 0.02 0.6 0.2 Invention B 0.1 0.2 1.5 0.7 0.005 0.007 0.02 0.010.01 0.2 Invention C 0.1 1.5 1.5 0.03 0.005 0.006 0.002 Invention D 0.051.4 2.3 0.3 0.005 0.007 Invention E 0.1 1.5 0.5 0.2 0.004 0.006Invention F 0.1 0.1 1.4 0.4 0.006 0.003 Comp. ex.

TABLE 2 Processing Annealing condition no. temp. (° C.) PH₂O/PH₂ Remarks1 700 0.01 Invention ex. 2 700 0.0004 Comp. ex. 3 800 0.01 Invention ex.4 800 0.03 Invention ex. 5 800 0.0004 Comp. ex. 6 800 0.0003 Comp. ex. 7900 0.02 Invention ex. 8 900 0.0004 Comp. ex.

The molten zinc plating bath was adjusted to a bath temperature of 500°C. and a bath composition of Al of 0.1 wt % and the balance of Zn andunavoidable impurities. The atmosphere of the reducing furnace wasadjusted to a ratio of the steam partial pressure and hydrogen partialpressure (PH₂O/PH₂) by introducing steam into N₂ gas to which H₂ gas isadded in an amount of 10 wt % to adjust the amount of introduction ofsteam. The annealing temperature and PH₂O/PH₂ were set to the valuesshown in Table 2, each of the steel sheets shown in Table 1 wasrecrystallization annealed, then was dipped in the plating bath. Theamount of plating was adjusted to 60 g/m² by nitrogen gas wiping. Thealloying treatment was performed by heating the steel sheet in N₂ gas at500° C. and holding it for 30 sec.

The strength of the steel sheets was evaluated by JIS Z 2201. 490 MPa ormore was judged as passing. The elongation of the steel sheets wasevaluated by obtaining a JIS 5 tensile test piece and performing anordinary temperature tensile test at a gauge thickness of 50 mm and atensile rate of 10 mm/min. A sheet exhibiting an elongation of 30% ormore was judged as passing.

The oxide particles in the plating layer were evaluated by polishing thecross section of the plating layer to expose it and observing it andcapturing an image of the oxide particles by a scan electron microscope(SEM). The image captured by the SEM was digitalized and the parts witha brightness corresponding to the oxides were extracted by imageanalysis to prepare a digital image. The prepared digital image wascleared of noise, then the equivalent circular diameters of theparticles were measured and the average value of the equivalent circulardiameters was found for the particles as a whole detected in theobserved field.

The extent of Fe—Zn alloying of the plating layer was evaluated byrandomly selecting 10 analysis points at each steel sheet andquantifying the concentration of Fe in the plating layer by glowdischarge optical emission spectrometry. The size of each analysis pointwas made a constant diameter of 5 mm. When there are at least ninelocations where the concentration of Fe in the plating layer is 7 to 15wt %, a sheet is judged to pass, while in other cases, it is judged tofail. When there are two or more locations where the concentration of Fein the plating layer is less than 7 wt %, it is judged that the alloyingis insufficient and the sheet has failed, while when there are two ormore locations where the concentration is over 15 wt %, it is judgedthat the alloying is excessive and the sheet has failed.

Table 3 shows the results of the evaluation. From Table 3, the testmaterials subjected to the alloying molten zinc plating which passed instrength, elongation, and alloying degree were all examples of thepresent invention. The comparative examples either passed in thestrength and elongation, but failed in alloying degree or passed inelongation and alloying degree, but failed in strength. Further, it wasconfirmed that the plating layers in the test materials subjected to thealloying molten zinc plating of the examples of the present inventioncontained oxide particles of at least one type of oxides comprised of anAl oxide, Si oxide, Mn oxide, Al and Si complex oxide, Al and Mn complexoxide, Si and Mn complex oxide, or Al, Si, and Mn complete oxide.

TABLE 3 Average Test Treatment size of oxide Evaluation materialcondition particles in Evaluation Evaluation of alloying code numberplating layer of strength of elongation degree Remarks NA 3 0.2 P P PInvention ex. NA 4 0.4 P P P Invention ex. NA 5 ND P P F Comp. ex. NA 70.4 P P P Invention ex. NA 8 ND P P F Comp. Ex. A 3 0.4 P P P Inventionex. A 4 0.2 P P P Invention ex. A 5 ND P P F Comp. Ex. A 7 0.2 P P PInvention ex. A 8 ND P P F Comp. Ex. B 1 0.3 P P P Invention ex. B 2 NDP P F Comp. Ex. B 3 0.2 P P P Invention ex. B 4 0.2 P P P Invention ex.B 5 ND P P F Comp. Ex. B 6 ND P P F Comp. Ex. C 1 0.5 P P P Inventionex. C 2 ND P P F Comp. Ex. C 3 0.5 P P P Invention ex. C 4 0.5 P P PInvention ex. C 5 ND P P F Comp. Ex. C 6 ND P P F Comp. Ex. C 7 0.4 P PP Invention ex. C 8 ND P P F Comp. Ex. D 3 0.6 P P P Invention ex. D 40.5 P P P Invention ex. D 5 ND P P F Comp. Ex. D 6 ND P P F Comp. Ex. E3 0.2 P P P Invention ex. E 4 0.2 P P P Invention ex. E 5 ND P P F Comp.Ex. E 6 ND P P F Comp. Ex. F 3 ND P F P Comp. Ex. F 4 ND P F P Comp. Ex.F 5 ND P F P Comp. Ex. F 6 ND P F P Comp. Ex. P: pass, F: fail, ND: notdetected.

INDUSTRIAL APPLICABILITY

The alloyed molten zinc plated steel sheet of the present invention is asteel sheet which contains oxide particles in the plating layer, wherebythe area of the unformed parts of the Fe—Zn alloy phase becomes lessthan 10% of the area of the steel sheet as a whole and the strength andshapeability become superior. According to the process of production ofthe present invention, it is possible to produce this at a low cost byjust changing the operating conditions of an existing continuous zincplating production system.

1. An alloyed molten zinc plated steel sheet characterized by comprisinga steel sheet including, by wt %, C: 0.05 to 0.40%, Si: 0.2 to 3.0%, andMn: 0.1 to 2.5% and further including at least one of: P: 0.001 to0.05%, S: 0.001 to 0.05%, Al: 0.01% to 2%, B: 0.0005% to less than0.01%, Ti: 0.01% to less than 0.1%, V: 0.01% to less than 0.3%, Cr:0.01% to less than 1%, Nb: 0.01% to less than 0.1%, Ni: 0.01% to lessthan 2.0%, Cu: 0.01% to less than 2.0%, Co: 0.01% to less than 2.0%, Mo:0.01% to less than 2.0%, with the balance comprised of Fe andunavoidable impurities, having on its surface a Zn alloy plating layercomprised of Fe in a concentration of 7 to 15 wt %, Al in aconcentration of 0.01 to 1 wt %, and the balance of Zn and unavoidableimpurities, said plating layer containing oxide particles of at leastone type of oxide selected from the group consisting of an Al oxide, Sioxide, Mn oxide, Al and Si complex oxide, Al and Mn complex oxide, Siand Mn complex oxide, and Al, Si, and Mn complex oxide alone or incombination, and an average diameter of the particle size of said oxideis 0.01-1 μm.
 2. An alloyed molten zinc plated steel sheet as set forthin claim 1, characterized in that said oxide particles are comprised ofat least one of silicon oxide, manganese oxide, aluminum oxide, aluminumsilicate, manganese silicate, manganese aluminum oxide, and manganesealuminum silicate.
 3. An alloyed molten zinc plated steel sheet as setforth in claim 1, characterized in that the structure of said steelsheet has a complex structure of a ferrite phase, bainite phase, andresidual austenite phase.
 4. A process of production of an alloyedmolten zinc plated steel sheet described in claim 1 by a continuousmolten zinc plating system, said process of production of an alloyedmolten zinc plated steel sheet characterized by making a heatingtemperature T at a recrystallization annealing step in a reducingfurnace of said system 650° C. to 900° C., passing the steel sheetthrough an atmosphere where a ratio PH₂O/PH₂ of the steam partialpressure PH₂O and hydrogen partial pressure PH₂ of the atmosphere ofsaid reducing furnace is: 1.4×10⁻¹⁰T²−1.0×10⁻⁷T+5.0×10⁻⁴ to6.4×10⁻⁷T²+1.7×10⁻⁴T−0.1, forming internal oxide at a region from thesurface of the steel sheet to a depth of 1.0 μm, then successivelyperforming molten zinc plating treatment and alloying treatment.
 5. Aprocess of production of an alloyed molten zinc plated steel sheet asset forth in claim 4, characterized in that said oxide particles arecomprised of at least one of silicon oxide, manganese oxide, aluminumoxide, aluminum silicate, manganese silicate, manganese aluminum oxide,and manganese aluminum silicate.
 6. A process of production of analloyed molten zinc plated steel sheet as set forth claim 4,characterized in that the structure of said steel sheet has a complexstructure of a ferrite phase, bainite phase, and residual austenitephase.